bash-completion: remove hashbang

Hashbangs are meant for scripts that are executed, but a bash completion
script is meant to be sourced and therefor shouldn't have a hashbang.

Remarked by Debian's ``lintian`` tool with the
``bash-completion-with-hashbang`` tag.

etc/mpv.bash-completion

@@ -1,5 +1,3 @@
-#!/bin/bash
-
 #
 # This file is part of mpv.
 #